Purified antithrombin III (AT3) will be labelled with radioactive iodine without denaturation (*I). It is proposed to apply kinetic studies of *I-AT3 in normal controls and patients with hereditary and acquired AT3 deficiencies in order to determine: (1) whether in vitro behavior of their AT3 is normal or abnormal; (2) whether their low plasma levels are due to reduced synthesis or increased consumption of AT3; and (3) what effects heparin and coumadin treatment have on synthesis and consumption of AT3. Additional in vitro studies with the purified AT3 will be performed in order to determine functional aspects of the AT3 molecule from normals as compared to the patient group.